In the industries of thermal power or heat generation, coal mines, raw coal application, ports and wharf, metallurgy, building material, and the like, the impurities in granule materials need to be removed. Taking thermal power generation for example, due to various reasons, raw coal, in the process of production, transportation, storing and using, is mixed with iron, wood blocks, stones, cloth, ropes, fibers, weeds and plastics, which easily cause a belt conveyor and raw coal outlet to be blocked. Especially at the sequential stage of producing powder, the impurities easily cause a milling machine to be damaged and a separator to be blocked. A smooth supply of powder cannot be guaranteed. Not only frequent cleaning costs money, but it also affects safe and stable operation of the boiler